The world is currently experiencing a severe environmental crisis due to high atmospheric carbon dioxide levels, in addition to an impending energy shortage. Both of these problems are a direct result from excessive fossil fuel utilization. The use of hydrogen (H2) as a fuel offers one highly attractive solution to these problems since the combustion of hydrogen (in a fuel cell, for example) produces only water and hence is free of emissions containing carbon and other pollutants.
Hydrogen gas contains more energy per mass unit than any known combustible fuel (approximately three times as much as gasoline). However, the use of hydrogen as a fuel has some drawbacks, since under ambient conditions it is an extremely low density gas. Hydrogen can be liquefied, cryogenically or with very high pressures (b.p., 1 atm.: −253° C.). These extreme conditions limit the potential widespread and large scale use of hydrogen.
A simple solution to these problems is to store the hydrogen chemically in a material that contains a very high weight percentage of hydrogen in a system that is capable of releasing the hydrogen on demand. Various compounds are known that contain a high gravimetric amount of hydrogen. Many of these compounds (e.g. lithium hydride (LiH, 12.8 wt % hydrogen) or lithium aluminum hydride (LiAlH4, 10.6 wt % hydrogen)) or alane (AlH3, 9 wt % hydrogen, Sandrock, G.; Reilly, J.; Graetz, J.; Wegrzyn, J. “Activated Aluminum Hydride Hydrogen Storage Compositions and Uses Thereof” U.S. Patent Application Publication No. 2007-0025908) suffer from being highly reactive or even potentially explosive and thus it is unlikely that commercial applications utilizing these compounds will be developed. Hydrogen generation by hydrolysis of magnesium hydride (MgH2) is a safer method but usage of metal hydrides will be problematic due to their inherent water sensitivity (U.S. Pat. No. 5,198,207, “Method for the Preparation of Active Magnesium Hydride-Magnesium Hydrogen Storage Systems, Which Reversibly Absorb Hydrogen”, Wilfried Knott, Klaus-Dieter Klein, Gotz Koerner, Th. Goldschmidt A G, Oct. 30, 1991) which can easily cause hydrogen formation under unwanted conditions.
The relatively benign compound ammonia borane (AB═NH3BH3, 19.6 wt % hydrogen) has been indicated by the United States Department of Energy to be number two on the list of potential hydrogen storage materials in terms of hydrogen content; the number one material is methane (CH4), combustion of which will lead to the obvious problem of further carbon dioxide emissions. Clearly, AB has significant advantages over all other known materials in applications where hydrogen is required as an energy carrier.
The United States Department of Energy has funded a significant amount of research, summarized below, that is relevant to this work. For example, a very efficient homogenous iridium catalyst for the dehydrogenation of AB was discovered by Goldberg's group which demonstrates fast release of H2 gas within 20 minutes at room temperature (Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048-12049). However, the system can only generate one equivalent of hydrogen and the insoluble boron-nitrogen containing materials (borazanes) that are formed are very difficult to recycle. Baker and co-workers described the acid initiation of AB dehydrogenation for hydrogen storage. (Stephens, F. H.; Baker, R. T.; Matus, M. H.; Grant, D. J.; Dixon, D. A. Angew, Chem. Int. Ed. 2007, 46, 746-749). However, their use of an air sensitive, strong Brønsted acid (e.g. trifluoromethanesulfonic acid) or strong Lewis acid (e.g. tris(pentafluorophenyl)borane) may prohibit their actual on-board applications. Baker has also reported the use of a homogeneous nickel containing catalyst that is capable of effecting ammonia borane dehydrogenation (R. J. Keaton, J. M. Blacquiere, R. T. Baker J. Am. Chem. Soc. 2007, 129, 1844). This highly air sensitive system exhibits very poor efficiency and requires heating to 60° C. for three hours to afford a 94% yield of hydrogen.
Several reports have appeared from Xu and coworkers regarding heterogeneous ammonia borane hydrolysis catalysts containing noble metals (a) Xu, Q.; Chandra, M. J. Alloys Cmpd. 2007, 446-447, 729-732; (b) Chandra, M.; Xu, Q. J. Power Sources 2007, 168, 125-142) or base metals ((a) Chandra, M.; Xu, Q. J. Power Sources 2006, 163, 364; (b) Man, J.-M.; Zhang, X.-B.; Han, S.; Shioyama, H.; Xu, Q. Angew. Chem., Int. Ed. 2008, 47, 2287). Mohajeri et al have reported a similar system using K2PtCl6 as a precatalyst (Mohajeri, M.; T-Raissi, A.; Adebiyi, O. J. Power Sources 2007, 167, 482-485) as well as using other noble metal catalysts (Mohajeri, M.; T-Raissi, A.; Bokerman, G. U.S. Pat. No. 7,285,142, issued Oct. 23, 2007). These systems all use relatively high catalyst loadings and are expected to exhibit limited air stability. Additionally, the systems produce nanoparticles as the active catalysts which have unknown and potentially problematic health effects. Manners has reported heterogeneous catalysts that are capable of hydrolyzing ammonia borane which contain cobalt, rhodium or iridium (Clark, T. J.; Whittel, G. R.; Manners, I. Inorg. Chem. 2007, 46, 7522). Again, the catalysts are relatively inefficient (cobalt), and require somewhat high catalyst loadings (rhodium, iridium). Xu and Chandra have demonstrated an ammonia borane hydrolysis system utilizing the solid acids Amberlyst or Dowex (Chandra, M.; Xu, Q. J. Power Sources 2006, 159, 855-860, Japan patent JP 2006213563). The acids need to be regenerated periodically to retain activity. Ramachandran et al have reported catalytic hydrogen generation from ammonia borane via methanolysis (Ramachandran, P. V.; Gagare, P. D. Inorg. Chem. 2007, 46, 7810, WO 2007106459). A non-catalytic system has been reported by Varma and coworkers which contains ammonia borane, aluminum powder and water (Diwan, M.; Diakov, V.; Shafirovich, E.; Varma, A. Int. J. Hydrogen Energy 2008, 33, 1135-1141) and produces hydrogen via a combustion process. The system is expected to be problematic owing to the highly exothermic and potentially dangerous reaction between aluminum powder, water and ammonia borane, in addition to its non-catalytic nature. Xu and coworkers have demonstrated an electrochemical cell that utilizes a platinum or gold electrode for direct oxidation of ammonia borane (Zhang, X.-B.; Han, S.; Yan, J.-M.; Chandra, M.; Shioyama, H.; Yasuda, K.; Kuriyama, N.; Kobayashi, T.; Xu, Q. J. Power Sources 2007, 168, 167-171). The authors note that the efficiency of the system must be improved before usage is feasible. Sneddon and co-workers have reported a heterogenous system using ammonia triborane (NH3B3H7, 17.9 wt % hydrogen) and rhodium for chemical storage of hydrogen (Yoon, C. W.; Sneddon, L. G. J. Am. Chem. Soc. 2006, 128, 13993). However, ammonia triborane requires more synthetic steps and contains a lower gravimetric amount of hydrogen than ammonia borane and these facts will limit the application of ammonia triborane as a hydrogen storage material. Another ammonia borane derivative that has been reported recently is MNH2BH3 (M=Li or Na). The material releases approximately 11 wt % (M=Li) or 7.5 wt % (M=Na) upon heating to 90° C. over ca. 19 hours (Xiong, Z.; Yong, W. K.; Wu, G.; Chen, P.; Shaw, W.; Karkamkar, A.; Autrey, T.; Jones, M. O.; Johnson, S. R.; Edwards, P. P.; David, W. I. F. Nature Materials 2008, 7, 138). The rate of hydrogen release will need to be improved upon dramatically before this system could be commercialized. A closely related material is Ca(NH2BH3)2 (Diyabalanage, H. V. K.; Shrestha, R. P.; Semelsberger, T. A.; Scott, B. L.; Bowden, M. E.; Davis, B. L.; Burrell, A. K. Angew. Chem. Int. Ed. 2007, 46, 8995-8997). Heating this material results in hydrogen release, although it suffers from the same problem as MNH2BH3 (M=Li or Na) in that the process occurs over a long time period. Other heterogenous systems that have been investigated include the use of polyhedral borane anion salts (such as B11H14−, B12H122− and B10H102−) and rhodium boride (RhB), but these studies are still very preliminary (Hawthorne, M. F. et al. “Chemical Hydrogen Storage Using Polyhedral Borane Anion Salts”, FY 2006 Annual Progress Report, DOE Hydrogen program, IV.B.4f, 416-417). The use of anionic polyhedral borane salts will be problematic since their cost will be significantly higher than ammonia borane. Another material that is used for hydrogen generation via transition metal catalyzed hydrolysis is sodium borohydride (NaBH4; 10.7 wt % hydrogen; Amendola et al, U.S. Pat. No. 6,534,033 granted to Millennium Cell). Sodium borohydride suffers from water sensitivity and contact of these two materials can result in hydrogen liberation in circumstances where it was not intended, dramatically increasing the risk of fire or explosion. Compared to sodium boroyhydride, ammonia borane forms relatively stable aqueous solutions and hence the risk of fire with ammonia borane is much lower than with sodium borohydride. Heterogeneous transition metal-catalyzed dissociation and hydrolysis of AB has been reported (Chandra, M.; Xu. Q. J. Power Sciences, 2006, 156, 190-194).